It has perhaps become something of a cliché to regard the attacks of 11 September 2001 and subsequent events as a seismic moment in not only the fight against terrorism but also the way in which we perceive it. Perhaps once thought of as the collective term for individual acts of violence committed by groups of aggrieved individuals against a single state for a particular ideological cause, terrorism has since come to be perceived as something of a global phenomenon affecting all states in one form or another. This change in perception has led to the United Nations becoming more involved in tackling the issue at a systemic level, with the Security Council making greater demands of states and requiring increased co-operation in regard to a wide range of terrorist-related issues and the General Assembly addressing terrorism in a more general sense as opposed to specific manifestations of it. Furthermore, in the military context this globalization of the terror threat has led to the notion of a global ‘war on terror’ becoming prominent in some quarters.

In the contemporary environment states are also under an obligation to refrain from participating in, to prevent, and to punish terrorism like never before. The regulation of states’ responsibility for failing to abide by these obligations – which are of both a positive and a negative nature – are fraught with difficulties, not least of all due to the covert nature of the terrorist activity concerned and because the capabilities and intentions of states are so varied. It is this general facet of the phenomenon that Kimberley Trapp in State Responsibility for International Terrorism focuses on. Although other relatively recent works have addressed state responsibility in the context of terrorism,Footnote ¹ Trapp's work takes a novel approach in that, as the author herself identifies, ‘[w]hile studies on particular aspects of state responsibility for international terrorism have been undertaken following the adoption of the ILC Articles on State Responsibility, there has yet to be a comprehensive assessment of the way in which state responsibility for international terrorism is or might be implemented’ (p. 2).

The comprehensive assessment that Trapp aims for in the book is realized through an exploration of ‘the primary and secondary rules of state responsibility, with a view to identifying both the problems and prospects for effectively holding states responsible for international wrongful acts related to terrorism’ (p. 2). The book thus essentially aims to offer an evaluation as to whether sweeping changes to these existing rules are necessary to address the issue of terrorism. In concluding that no significant changes to the rules themselves are necessary, Trapp highlights how they have been perceived or used incorrectly and offers up ways in which they could be more appropriately used in the terrorism context. In this respect an overriding theme of the book is that it does not portray terrorism as a new challenge which renders the existing rules deficient or antiquated and neither does it seek to set out a completely new approach to state responsibility for international terrorism. Instead, Trapp takes a thoroughly pragmatic approach in exploring the existing rules and their potential (or ‘prospects’) for holding states responsible for their role in terrorism and terrorist acts. It is in this sense that the book self-consciously maps ‘the mechanisms available under international law for implementing a state's responsibility with a view to laying the groundwork for successful invocations of state responsibility for international terrorism’ (p. 267).

The book opens with a chapter (Chapter 1) that sets out the legal context within which state responsibility for international terrorism is examined, such as the framework of the law of state responsibility and the regimes of accountability and substantive law applicable to international terrorism. It also addresses the sticky issue of the definition of terrorism, which, of course, is of vital importance in understanding what exactly states are being held responsible for. However, this is also an issue that has received dedicated treatment in other scholarly worksFootnote ² and, in order to avoid getting bogged down in such debates, Trapp takes the sensible approach of adopting a working definition for the purposes of the book, which is one generally drawn upon in the relevant terrorism suppression conventions and in the Report of the High-Level Panel on Threats, Challenges and Change of 2004, which Trapp argues reflects the emerging international consensus regarding such definitional issues. Setting out her working definition early on in the book in this way and then sticking to it means that the issue does not dominate or impede subsequent discussion.

Although the book is not formally divided into sections, the chapters take on a logical flow as they weave through the relevant primary and then secondary obligations. Chapter 2 thus assesses the forms of state participation in international terrorism that are prohibited in international law and the means by which the conduct of non-state actors may be attributed to them. While this particular chapter addresses the obligations that treat the state as the direct subject of control, Chapter 3 assesses the obligations whereupon the state is treated ‘as the mechanism of control through which individual terrorist behaviour is addressed’ (p. 62); that is, the obligation to prevent acts of terrorism and the obligation to extradite or submit to prosecution individuals that are responsible for them.

A breach of the rules discussed in Chapters 2 and 3 gives rise to a state's responsibility for an internationally wrongful act. The wrongdoing state is thus under a secondary obligation to cease the wrongful conduct and to make full reparation for any injury caused. Although states have yet to utilize the International Court of Justice (ICJ) to formally implement another state's responsibility for terrorism, Chapter 4 nonetheless specifically addresses the prospects for the Court's jurisdiction over disputes relating to state responsibility for international terrorism. In the absence of a binding determination by the ICJ in this context and other institutional remedies, states are left to rely on other mechanisms. While the forcible mechanism is addressed in Chapter 2 – that is, the use of force in self-defence – ‘[s]uch a response, however, operates on the basis of a security paradigm, and does nothing to restore the primary legal relationship as between the insured and wrongdoing states’ (p. 182). As such, Chapter 5 addresses both the non-military measures adopted in response to a breach of international terrorism obligations under general international law with the aim of securing a wrongdoing state's compliance with its secondary obligations, such as acts of retorsion, countermeasures, and necessary measures leading to a preclusion of wrongfulness, and the primary and secondary rules of the various subsystems of international law of diplomatic relations, trade obligations, and air service arrangements.

Lastly, while the book's focus is that of state responsibility for terrorism, given that the traditional paradigm of enforcement has been on the individual criminal level, Chapter 6 provides an interesting discussion on the relationship between individual criminal responsibility and state responsibility in the terrorism context.

Although the book as a whole covers novel ground in what is a comprehensive coverage of the law and legal remedies in this area, it is perhaps the content of Chapter 2 on the law of state terrorism and the question of attribution which is most engaging and – it has to be said – controversial, particularly as this has received perhaps most attention from the scholarly community, including Trapp herself,Footnote ³ and which arguably best encapsulates the subtitle of the book of ‘problems and prospects’. It is here that Trapp most evidently presents her defence of the existing rules of responsibility in their applicability to terrorism by rejecting from the outset the existence of a lex specialis prohibiting acts of terrorism by states. With this the author succinctly and ably demonstrates how the traditional legal notions of aggression, force, and intervention apply to the sphere of international terrorism.

While the chapter's short coverage of the prohibition of state terrorism does not contain anything substantively new, the following section on the rules regarding the attribution to states of the actions of non-state actors in a sense aims to put these rules through a stress test in an inquiry as to whether they ‘are adequate to respond to the threat posed by state participation in acts of international terrorism’ (p. 34). It is in this light that she argues that the ‘effective control’ standard, as established and reaffirmed by the ICJ in the Nicaragua and Bosnia Genocide cases is not the appropriate one in the context of terrorism. Instead, Trapp argues that the ‘overall control’ standard, which was first brought to light by the Appeals Chamber of the ICTY in the Tadić case but subsequently rebutted by the ICJ in the Bosnia Genocide case, can ‘respond to the particularities of the terrorism context in a way that rigid adherence to the Nicaragua standard does not’ (p. 45). This argument is supported by the author's claim that Article 8 of the ILC Articles on State Responsibility is drafted broadly enough to incorporate this standard. This is a progressive and thought-provoking argument in that it has to be acknowledged that ‘state support for terrorism, no matter how substantial, will not generally satisfy the “effective control” test’ (p. 44). In this respect while Trapp does not agree with the current perception of the applicability of the rules in this context, which is generally restricted to the ‘effective control’ standard since the ICJ's jurisprudence in the Bosnia Genocide case, she nonetheless believes that the rules themselves are up to the job of dealing with issues of state responsibility in the terrorism context. The only drawback with Trapp's analysis here is that while the suitability of the ‘overall control’ standard in the terrorism context is set out, the section does not subsequently go into any real detail as to what it involves or its operational prospects. This would have produced a significant addition to what is on the face of it a strong argument.

However, Trapp goes on to examine the challenge from some within the academic community to these broadly accepted bases for attribution as they apply to terrorism. Indeed, the contention that states can be held responsible in the jus ad bellum context through complicity or acquiescence in the ‘armed attacks’ of terrorists based within their territory and therefore be subject to a response in self-defence has recently been made by some on the basis of the overwhelming acceptance by the international community of the US's invocation of what can be described as a ‘harbouring’ standard of attribution in the aftermath of the 9/11 attacks. On this occasion, and as is well known by now, this standard of attribution led to the use of force in self-defence being employed by the US both against al Qaeda, which was located in Afghanistan and held to be responsible for the attacks, and against the Taliban, which was the de facto government of the country, as it was deemed to be complicit in the attacks by harbouring al Qaeda, including by refusing to hand over members of the organization upon a request by the US to do so.

Trapp acknowledges the near universal acceptance of the invocation of such a standard of attribution on this occasion. Yet she proceeds to dismiss the claim that this reaction has led to the rules of attribution being watered down in the jus ad bellum context in order to incorporate a state's complicity or acquiescence in the activities of a non-state actor to permit forcible action in self-defence to be taken by a victim state against the non-state entities and the host state. Indeed, through an examination of incidents such as Israel's armed response within Syria to the terrorist attack in Haifa in 2003 by Islamic Jihad, the Israeli incursion into Lebanon in 2006 following the abduction of two Israeli soldiers and the launching of rockets into Israel by Hezbollah, Turkey's various incursions into Iraqi territory in response to attacks by the PKK, and Columbia's targeted military operations in Ecuador in response to attacks by FARC, Trapp demonstrates – successfully in this reviewer's opinionFootnote ⁴ – that ‘[i]n the post-9/11 world, Operation Enduring Freedom remains the only case in which the international community accepted a state's right to use force in self-defence against both non-state terrorist actors and the state from whose territory such terrorist actors operated, when the terrorist attack being responded to was not attributable to the territorial state’ (p. 54). Instead, Trapp argues that the responses in self-defence have been limited solely to targeting the non-state actors responsible for the attacks in the host state. While it may be questioned whether enough state practice is drawn upon in the book to support the arguments made, the practice is not so much used to construct an argument that a new form of self-defence exists as to rebut the arguments of others as to the existence of one. In this regard, the practice seems more than sufficient. Yet there is also policy reasoning behind Trapp's arguments in that she acknowledges that the real difficulty with uses of force in self-defence against the host state based upon the attribution of the attacks of the non-state actors to them through complicity ‘is that targeting a terrorist-harbouring state directly is likely to decrease its counter-terrorism capacity’ (p. 60). However, the author does not also note the potentially significant impact upon international peace and security that such uses of force have. This surely is a far greater concern.

The subsequent practice under focus is not, however, without significance. Indeed, ‘post 9/11 practice confirms that a use of force directly targeting non-state terrorist actors who have launched an armed attack is a legitimate exercise of the right of self-defence’ (p. 51), something which for a long time was – and some would say still is – controversial. Arguing against an unnecessarily restrictive reading of the ICJ's jurisprudence in order to make attribution essential for the invocation of self-defence in response to an ‘armed attack’ and despite an acknowledged failure by the Court to directly address the issue, Trapp argues that ‘there is a right under international law to use force directly against non-state terrorist actors operating from foreign territory. An attribution-based reading of “armed attack” under Article 51 of the UN Charter should therefore be laid to rest’ (p. 59).

Central to Trapp's arguments in this respect is the point that such an invocation of attribution is unnecessary as the action can be justified on the established requirements of customary international law of necessity (and proportionality).Footnote ⁵ Indeed, if the host state is either ‘unable or unwilling’ to take action against the non-state actors concerned it therefore becomes necessary for the victim state to take action. However, while the concern regarding the subjective nature of determinations that a state is ‘unable or unwilling’ to effectively prevent its territory from being used as a base for terrorist operations is acknowledged in that Trapp notes that ‘[s]uch uses of force amount to the substitution (and imposition) of the victim state's views on how to respond to terrorist threats emanating from the host state's territory for those of the host state’ (p. 59), the book does not then go on to offer any suggestions as to how this subjectivity could be overcome. Indeed, what is to happen when the acting state believes that a host state is unable or unwilling to take action but other states do not? Could such situations be determined by the UN Security Council? Or the ICJ? Or do we simply have to live with such critical subjective determinations? Again, engaging with such questions would have rounded off what are in other respects solid arguments.

All in all, Trapp has produced a tour de force on the contours of the contemporary legal debates and standing of state responsibility for international terrorism. The book manages to weave much detail and analysis amongst the clearly argued and accessible paragraphs that make up this well-structured monograph. The stated aim of providing a comprehensive coverage of the area has on the whole been achieved, with regime interaction being a notable theme throughout. The arguments are portrayed in an intelligent yet accessible style that opens up the book's readership to students, academics, lawyers, and government advisers. This comes at a particularly poignant moment now that the debates regarding the impact of the events of 9/11 have had a chance to settle.